U.S. Pat. No. 4,629,676 (Hayakawa et al) and European Patent No. 0174634 (Fuji Photo Film Co., Ltd.) disclose an image forming method comprising the steps of imagewise exposing to light a light-sensitive material and heating the light-sensitive material. The light-sensitive material comprises a support and a light-sensitive hardening layer containing silver halide, a reducing agent and a polymerizable compound. The silver halide is developed by heating the light-sensitive material to form a polymer image. The polymerization is initiated by an oxidation radical of the reducing agent (including a radical formed from an oxidation product of the reducing agent).
The above image forming method can be applied for preparing a lithographic plate. In that case, the strength of the hardened image is particularly required. In the above light-sensitive material, silver halide, a reducing agent and a polymerizable compound are incorporated in a single light-sensitive hardening layer. However, it is very difficult for the single hardening layer containing various components to give a hardened image of enough strength for a lithographic plate.
U.S. Pat. No. 5,122,443 (Takeda), U.S. Pat. No. 5,290,659 (Takeda) and European Patent No. 0426192 (Fuji Photo Film Co., Ltd.) disclose embodiments of the light-sensitive materials advantageously used for the preparation of a lithographic plate. In those materials, a light-sensitive layer containing silver halide is separated from a hardening layer containing a polymerizable compound or a cross-linkable polymer. The hardening layer or the light-sensitive layer further contains a reducing agent. In view of the strength of the hardened image, the reducing agent is preferably contained in the light-sensitive layer.
A process for the preparation of a lithographic plate comprises the steps of imagewise exposing the light-sensitive material to light, heating the material, removing the light-sensitive layer from the material, and removing the unhardened area of the hardening layer with an alkaline etching solution. The formed replica image is used as a printing plate.